Electromagnet Core Material

I realize that soft iron is the most common material for this but I am wondering if there is something that will increase the magnetic strength.
Here is what I have.
Two electromagnetic coils in series connected by a yoke with a capacitor in parallel (for spark reduction). Cores are soft iron 3/8" or 5/16" x 1 1/4". The magnets move an armature bar that switches the circuit on/off 80-160 Hz.

I'm am looking for a material that will create a stronger magnetic field without having to increase voltage (3-9 volts DC < 1amp) .

What would the most efficient materials be that are available in those dimensions if one were willing to spend more $.